The contamination of soils with lead (Pb) and arsenic (As) has created potential health hazards at numerous sites throughout the country, including hundreds now designated as Superfund sites. The bioavailability (i.e., exposed to Pb and As from soil currently use assumptions based on bioavailability data from animal or in vitro models. Using the technique of stable Pb isotope dilution, we have developed a model for estimating soil Pb bioavailability in humans. This model examines changes in the ratio of 206Pb to 207Pb in blood, following the ingestion of trace quantities of Pb-contaminated soils. Thus, we propose to examine the human bioavailability of soils from a mining site, a smelter site, and a urban site. Through collaboration with EPA, we have already obtained soils from a mining site and a smelter site which are isotopically ideal for study. In addition, we propose to determine whether soil amending agents currently being tested in the field by EPA actually reduce soil Pb bioavailability in humans. This phase of our work is important because the same pre- and post-amended soils, from a former smelter site in Joplin, Missouri, are currently being evaluated in swine, rat and in vitro models, possibly allowing validation of the less expensive in vitro model. Finally, we will explore the possibility that the ingestion of these soils, some of which contain trace quantities of As, may allow us to estimate the urinary excretion fraction (UEF) for soil As. Collectively, these studies promise to improve the precision of risk assessments At site contaminated with Pb and As.